433 Mhz dipole vs. GPS: total decimation

I wanted to do long range telemetry so I got myself a dipole antenna for the modem. 

I never could check the range because the plane couldn't get a gps lock anymore, with the modem powered on. With the modem powered off, I had a 3D GPS lock, 17 sats in seconds.

What can I do to get both long range telemetry and good GPS?

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    • Not proven yet - Anton had good GPS fix with the SAME dipole, with the rest of the items shielded and powered separately. That already implies it is not the Dipole or its matching...Lets not confuse the issue here with unproven concepts. 

      Anton, you need to complete the test as I have asked, and all will be revealed...

      Joe

      • Not proven, like the actual frequencies that the telemetry system is utilizing and where the third harmonic is in relation to the L1 and L2 GPS frequencies (1575mhz and 1227mhz), the L2  landing around 424mhz and fourth order L1 around 393mhz respectively. However, a badly designed antenna as was shown, being poorly coupled into it's source greatly enhances parasitic signal interference in the entire system, not just the module itself, The first logical place to start solving interference problems it at the source, especially when known, and from that standpoint the most common cause of mysterious interference is not actually what is radiated by the antenna's elements, but what doesn't get radiated effectively and (the energy must go somewhere) gets propagated back through the 'ground' wires in the circuit and the entire system becomes an inefficient antenna. That is the start, get the antenna working as an antenna, radiating as much of the signal as is possible, and all the extra shielding and baluns are suddenly a lot less important. From there, also mounting the antenna vertically will reduce the saturation within the GPS antenna's field, by altering the radiating pattern effectively off axis to the gps, and on axis to a vertical ground station antenna, Next, ensure there are no ground loops within the system, if they are unaviodable, use baluns there, but a properly designed star point ground will often do wonders to eliminate contamination. Ground loops, and even non ground (any wire forming a loop) becomes a crude transformer winding, converting any magnetic frequency within it's dimension into a current, that current is short circuited and produces sizeable current and so is easily re radiated quite effectively into wires running parallel to it, just like a transformer, it also adds itself to any voltage travelling trough the wire. This cross coupling make tracing the source of interference extremely difficult, as the source (loop) does not even need physical connection to the affected circuit. Every system I design has these fundamentals engineered in from the outset, it saves a lot of pain down the track, and improves performance all round. Rather than blaming hardware from the outset, I suggest correcting the known problems first, just my two cents worth.

        • The more I read about impedance matching the more I'm certain that that is the problem. The new antenna has a coax cable longer than 1/8 wavelength. And that is a problem:

          "A cable becomes a transmission line when it has a length greater than λ/8 at the operating frequency.

          For example, the wavelength of a 433-MHz frequency is:

          λ = 300/fMHz = 300/433 = 0.7 meters or 27.5 inches

          A connecting cable is a transmission line if it’s longer than 0.7/8 = 0.0875 meters or 3.44 inches. All transmission lines have a characteristic impedance (ZO) that’s a function of the line’s inductance and capacitance:"

          I don't know the impedance of the transmitter. Measuring it requires expensive equipment. Neither do I know the impedance of the antenna. If I find a friend with the right tools, I still have to build impedance matching circuitry. It this point it seems easier to buy a whole new modem system with matching antennas. I don't understand how youtube is full of antenna building videos and nobody meaures/matches impedance. Anyway, I ordered antenna raw materials and I am still looking for a lab with the right equipment.

          I must also say that I haven't taken the original antenna to it's full range. I'll fly with that one and see how far I get.

          • Hi Anton,

            Anton, I think you are over-complicating the issue. The issue of coax cable lengths is unimportant for this discussion - the fact that your coax is longer than 1/8 Lambda is not a problem. It can be any length desired and has no effect on the signal at all, other than losses in the cable, which will increase with length ( talking meters of length for significant loss).  If the antenna impedance is a match to the coax impedance, normally 50ohms for this application, and if the transmitter output impedance is also 50 ohms, then all is matched and the coax length is irrelevant. The impedance of your transmitter should be around 50 ohms - that is a standard in the industry to which mostl transmitters and receivers that couple via unbalanced feeder, ie, coax cable, are designed to.

            If you want to short-circuit the process then get an antenna that is known to work, maybe the one Alasdair proposes. If that does not work, then we can go back to the process of elimination.

            If on the other hand, you wish to learn more about why it does not work, and more about the antenna and how it works, then maybe have a look at these blogs of mine - Antenna design has been a 30plus year career for me, on commercial and Military Aircraft, and for a small handfull of Amateur Radio Space Satellites..as well as large UAV's 150kg and larger..

            The topics may be somewhat advanced, but there are elements that may interest you, and maybe even help!

            Joe

            http://diydrones.com/profiles/blogs/dipole-style-antenna-for-433mhz

            http://diydrones.com/profiles/blogs/some-more-antennae-and-when-is-...

            http://diydrones.com/profiles/blogs/antenna-feedpoint-matching-balu...

            http://diydrones.com/profiles/blogs/video-and-datalink-antenna-for-...

    • The Dragonlink system uses 433mhz, so any of the receiver (v2-v3) antennas will suit, they are sma, and as shown in my photo have a tiny matching transformer on the pcb. They are available in various lead lengths or direct mounting as my pic shows. They sell around $15, and come with straightening tubes, cable ties to attach tubes and the lead length doesn't affect price. I have tried many different antenna designs in different applications and really the half wave dipole, properly matched and tuned is unbeatable, it has excellent polar pattern, outstanding range, and below two gig, little problem with multipathing and dropout. After testing mine, both 433mhz and 1200mhz, the VSWR is extremely good, under 1.3:1 over +/-40mhz on 1200. The conversion efficiency is also excellent, with both elements radiating properly the field strength at 10mtr is higher than any commercial omni antenna I have tried. You could try making your own, there are plenty of design options out there, but without spending lots on test gear, you will be field testing your results without much yardstick.
    • Hi Alasdair,

      This is one of the more insightful answers! 

      So the feeder of my modem is matched for a lamda/4 monopole antenna and it goes haywire on a lambda/4 dipole? I found no 433 antennas on the site you mentioned. This article is about impedance matching, but I understand only half of it. http://www.radio-electronics.com/info/antennas/yagi/yagi-feed-imped...

      This site has a nice selection of 433 antennas. How do I know if any of them are matched?

      http://www.getfpv.com/catalogsearch/result/?cat=0&q=433+antenna

      Or should go and build my own antenna, get an SWR measurement device and match it?

      (Sadly, where I live 1.2GHz is an illegal frequency to use. So I'm stuck with 433 en 2.4. So no 1.2G video for me. :( )

  • I think you might try mounting your 433mhz Transmitter , or at least its dipole antenna on back of your plane.

    like in this picture.

    3702259184?profile=original

    move your GPS and RX in fuselage , they are both receivers. they wont bother each other. just move Tx away from Autopilot and RXs. and mount your antenna vertical.

    I always use a separate battery for electronics , other than flight battery.

    you might try this too.

    Hope this will help

  • An overview of my different tries so far. Will add more experiments as suggested in this thread.

    3702259521?profile=original

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